1. Field of the Invention
The present invention relates generally to a patterned medium capable of high-density recording, a method for fabricating the same and a method for evaluating the same, and more particularly to a patterned medium for which the characteristics are optimized of magnetic metal thin film to be micro-fabricated with magnetic dots, a method for fabricating the same and a method for evaluating the same.
2. Description of the Related Art
In recent years, research and development are advanced for patterned media as the next-generation magnetic recording media replacing the thin film media. A patterned medium is a medium in which nanometer-scale micro-magnets, i.e., the so-called magnetic dots are arranged regularly in an array form on a substrate and 0 (zero)/1 (one)-digital signals for which each one (1) bit is represented by the direction of magnetization of each one magnetic dot are recorded. Since each bit is physically separated in such a patterned medium, a noise accompanied with magnetic inversion, that is a problem with a continuous film medium, is not generated and each one dot can be micronized to the limit of the thermal stability of a magnetic particle. Therefore, an ultra-high-density magnetic recording exceeding one (1) terabit/square inch can be expected. As to the structure of the magnetic recording medium using a continuous film formed by conventional sputtering etc., in order to attain high-density recording, nonmagnetic material is intentionally caused to deposit between magnetic particles, or the film property is caused to form a columnar structure (cylindrical structure) using high-gas-pressure film-forming. In these cases, portions having no magnetism, i.e., defects are produced in the magnetic film. However, those portions do not interfere with recording and reproducing because the exchange bonding force works between magnetic particles.
On the other hand, as the methods for producing a patterned medium, an approach in which a magnetic continuous film formed on a substrate is processed into a dot-array shape and an approach in which magnetic material is embedded into a substrate on which a hole array is formed are suggested. In the former production method, a fine pattern is drawn using an optical approach on a resist mask made from organic matter and applied on the magnetic film, and the magnetic film is processed into magnetic dots taking into consideration the selective ratio of the magnetic film and the photo-resist. In this case, the magnetic film is formed into the magnetic dots by forming partitions on the continuous film. However, one (1) dot is required to have a single magnetism. Therefore, the material of the magnetic metal thin film formed by sputtering as the magnetic film for patterned medium is required to have no defect. Because a patterned medium is a kind of vertical magnetic recording medium, the same material as that of vertical magnetic recording medium using a conventional continuous film is employed for it. As an evaluation index for the magnetic characteristic of recording/reproducing of patterned media, the inversion of magnetism of the dots as an aggregate has been discussed so far. However, recently, in a publication (see “Shift to Higher Resolution of Magnetic Force Microscope and Observation of Magnetic Domain of High-Density Magnetic Recording Media”, Shunji Ishio, Excerpts from the 27th Academic Lecture Meeting of Japan Society of Applied Magnetic, 2003, pp. 365–366) noting the inversion of magnetism of each dot, it is reported that, from an evaluation of the structure of magnetic domains of patterned media using an MFM (Magnetic Force Microscope), it has been found that each of the dots is not independent, magnetic clusters, each including some dots1 are formed and each of the magnetic cluster as the unit has inversion of magnetism with a different coersive force. In this case, material fluctuation inherent in the continuous magnetic film before the processing has been pointed out based on the fact that the difference in coersive force between the dots could not be cancelled even when the effect of exchange interaction between adjacent dots as a result of the non-uniformity caused by the processing was excluded (see Japanese Patent Application Laid-Open Publication Nos. 2001-110050, 2003-203333 and 2000-298009).
However, as described above, in the cases of the conventional patterned media, because the same material as that of the continuous film of the vertical magnetic recording media for the magnetic metal film on which the magnetic dots are to be formed is employed, difference in magnetic inversion among individual dots becomes more remarkable as the dots become finer when local material fluctuation (fluctuation of density) is inherent in the magnetic metal film. Therefore, there is concern that actual recording/reproducing may be interfered with. In order to observe such fluctuation of material density of the dot array, there is an approach for measuring the density by optical means such as an X-ray for the continuous film in a state before processing into dots. However, the area radiated by an X-ray is in the order of several meters, therefore, only the average density within the area is measured. Therefore, even if the there is a difference of density in one (1) dot having a size in the order of several 10 nano-meters, it would be difficult to detect the difference using an X-ray.